The fact they are looking good after watering then drooping again soon afterwards, tells me the substrate is overly dry. Four litres is not unreasonable, however I would suggest giving them one litre every half hour, so that they get the 4 litres over 2 hours. This way if your substrate is over dry and the water is just running straight through the pot without being effectively absorbed, the staggered watering method can help to rehydrate the substrate. This way, the first litre gets "sucked" up properly and swells the soil a bit, before the next litre is given and due to the swollen soil, more of the second litre will be absorbed more effectively, the same thing happening with litres three and four. It is a bit like blowing up a balloon, each time you add a litre and let it get "sucked" up a bit, the further the moisture will penetrate the soil, until the soil is fully moistened from edge to edge and top to bottom, just like filling a balloon a bit more with each breath. After getting your soil evenly moistened, try to maintain an even moisture level that is not too wet or not too dry. This can be a bit tricky to work out and has a lot of variables, such as temps, air flow, the plants size and requirements etc, but with time and experience, you will master this skill. At first you may have to repeat the four litre trick a second or even third day in a row, until your plants are staying normal and not drooping by the next day and the entire volume of soil has become evenly re-moistened again. A good way to know when to water again is by literally sticking your finger in to the soil and if the top inch or so is dry, then it is time to water deeply until run off, once again, and water the whole surface of the pot, not just next to the plant, roots will not grow in dry soil, nor will nutrients be available. Sticking your finger into multiple spots in the pot can be a better overall way to make a determination to the dryness of the entire pot. Watering to run off will ensure that water has penetrated from top to bottom and can help prevent any mineral salts from the fertilizers, from building up in your soil.
Hope this helps,.... Organoman.